Simple answer. You have one person working on the code for the UI. It goes slow. So you hire two more. It goes faster, so you hire three. All of a sudden you have too many people developing overlapping code, one person is always waiting for the other two to finish the sections they are on, so they aren't doing anything. So in order to catch the waiting person up on the new code, it has to be explained to them. The effort of coordination and communication starts to slow the process down again. So you take that extra third person, and put them on something else. Now the two people working on the UI are working at a faster speed again.

Basically you can only have so many people working on any one thing before it starts to hinder the process. Coding isn't like doing laundry where more people get it folded faster. Its more like changing the breaks on a car. Four people a great, five not so much, ten is pointless.

Now look at FF. They may have 10 people working on it, they may have 100 people working on it. At the point where you have 100 people working on it, you can have a whole team dedicated to things like easter egg quests without slowing down the overall process.

See. Simple explanation.